1. Field of the Invention
The present invention relates to a bolometer type infrared sensor, and more particularly to a bolometer type infrared sensor in which a bolometer material having hysteresis in resistancexe2x80x94temperature characteristic is used.
2. Description of the Related Art
A bolometer type infrared sensor is a sensor in which infrared rays are irradiated to a bolometer material that is separated in heat from a substrate and in which the infrared rays are detected based on a resistance change which is caused by a temperature change.
As the bolometer material of such a bolometer type infrared sensor, it is preferable for the bolometer material to have a high temperature coefficient of resistance (TCR) %/K which is a resistance change percentage for every degree of temperature change. Materials having such a characteristic are reported in metal, metallic oxide, and a semiconductor. For example, in Japanese Laid Open Patent Application (JP-A-Heisei 5-206526) and U.S. Pat. Nos. 5,288,649 and 5,367,169, the technique is disclosed in which amorphous silicon (a-Si) doped into an n-type or a p-type semiconductor is used as the bolometer material. Also, vanadium oxide or a bolometer material in which vanadium oxide is used as a base bolometer material are often used for the bolometer. Those characteristics are reported in reference (Solar Energy Materials 14, 205 (1986)) and the reference (Physical Review B 22, 2626 (1980)).
The bolometer material of an a-Si system has a relatively high value of TCR of 3%/K but has a higher resistivity than 1000 xcexa9cm. When the resistivity is high, a large Johnson noise is generated when the resistance value of the diaphragm is read out. As a result, substantial sensitivity to the infrared rays is not increased so much. On the contrary, when the resistivity is very small, the influence of the wiring resistance appears and the high sensitivity is not attained. Therefore, the resistivity is desirable in a range of 0.01 to 1 xcexa9cm.
On the other hand, the resistivity is as relatively low as 0.1 xcexa9cm in the bolometer material composed of vanadium oxide or containing it as the base bolometer material and a sensor having a TCR value of about 2%/K has been obtained. However, to aim at the sensor having higher sensitivity, it is necessary for the sensor to have a larger TCR value. To attain such a larger TCR value by use of vanadium oxide, a way of using the phase transition of vanadium oxide could be considered. The resistance change of equal to or more than 2 digits is generally observed before and after the phase transition of vanadium oxide. Also, by doping various metal elements in vanadium oxide, the transition temperature can be controlled to a suitable temperature. For these reasons, a high sensitive infrared ray detection characteristic can be expected if vanadium oxide is used as the material of the bolometer.
However, it is known that the resistivity of vanadium oxide has a hysteresis to the cycle of the temperature change. Conventionally, the bolometer material having a hysteresis in the resistivity temperature characteristic could not be used for the infrared ray sensing unit for the following reasons.
FIG. 5 is a diagram illustrating a hysteresis curve of a bolometer material having a hysteresis in the resistivity temperature characteristic. In FIG. 5, it is supposed that a stable phase on a lower temperature side is a first phase and a stable phase on the higher temperature side is a second phase. In the following description, a point in the figure, i.e., a set of a bolometer temperature and a bolometer resistance corresponding to the bolometer temperature is shown as a state of the bolometer
A resistivity xcfx81 of the bolometer material in FIG. 5 changes as follows with the temperature change. A logarithm of the resistivity is indicated in FIG. 5.
First, a bolometer temperature is increased from the state a of the first phase. At this time, the resistivity changes gently until the bolometer temperature reaches a temperature corresponding to a critical state b. When the bolometer temperature is increased beyond the critical state b, the phase transition begins so that the resistivity decreases rapidly. Then, when the bolometer temperature reaches a temperature corresponding to a state c, the rapid change of the resistivity happens no longer, even if the bolometer temperature is increased. The resistivity changes gently again.
That is, the bolometer material undergoes the phase transition from the first phase to the second phase while the bolometer state changes from the state b to the states c. The phase transition is completed at the critical state c so that the bolometer state changes to the second phase which is the stable phase on the higher temperature side. The curve bc is referred to as a temperature increasing curve in the following description.
On the other hand, the bolometer temperature of the bolometer material is decreased from the state f in the second phase. At this time, even if the bolometer temperature reaches the temperature corresponding to the state c, the phase transition does not happen. Therefore, the bolometer material maintains a second phase. In this temperature region, the resistivity of the bolometer material changes gently. When the bolometer temperature is further decreased so that the bolometer temperature reaches a temperature corresponding to the state d, the phase transition begins so that the resistivity increases rapidly. The rapid increase of the resistivity continues to the temperature corresponding to state e. Then, when the bolometer temperature is further decreased to pass through the state e, the resistivity of the bolometer material changes gently in this temperature region.
The curve de is referred to as a temperature decreasing curve in the following description. The bolometer material performs the phase transition from the second phase to the first phase in the temperature region corresponding to the temperature decreasing curve.
The point to which an attention should be paid when the bolometer material is used is that the phase transitions shown by the temperature increasing curve and the temperature decreasing curve are a non-reversible process.
Now, it is supposed that the state of the bolometer material is changed from the state a of the first phase via the critical state b to one point p1 on the temperature increasing curve, and then the temperature is decreased. In this case, the state of the bolometer material does not change along the temperature increasing curve from state p1 to the critical state b in an opposite direction. The state of the bolometer material changes from state p1 to the lower temperature side in approximately parallel to curve a-b. That is, the state of the bolometer material changes from the point p1 to the left side in FIG. 5.
Then, while the state of the bolometer material reaches the state shown by the point p in FIG. 5 via the above process, the resistivity gently changes along the curve p-p1 in FIG. 5, if the temperature of the bolometer material is increased again. Thereafter, when the state of the bolometer material reaches the point p1 on the temperature increasing curve, the resistivity begins to change rapidly along the temperature increasing curve.
The similar phenomenon occurs in the bolometer material when the temperature is decreased from the state f in the second phase.
Now, it is supposed that the state of the bolometer material changes from the state f in the second phase to a state p2 on the temperature decreasing curve via the critical state d and then the temperature is increased again. In this case, the state of the bolometer material does not change from the state p2 to the critical state d on the temperature decreasing curve in the opposite direction. The state of the bolometer material changes from the state p2 to a point on the higher temperature side in approximately parallel to curve d-f. That is, the state of the bolometer changes from the point p2 to the right side in FIG. 5.
Then, when the state of the bolometer material reaches a point shown by the point p in FIG. 5 via the above process, the resistivity gently changes along the curve p-p2 in FIG. 5, if the temperature of the bolometer material is decreased again. Thereafter, the state of the bolometer material reaches the point p2 on the temperature decreasing curve, so that the state of the bolometer material begins to change rapidly along the temperature decreasing curve.
Therefore, to attain high infrared ray detection sensitivity by use of the phase transition, it could be considered that the bolometer material having the physical chemistry structure corresponding to the one p1 on the temperature increasing curve is used. That is, it could be considered to use the bolometer material having a crystal structure and crystal incompleteness when the bolometer material is a single crystal, and the crystal structure, the crystal incompleteness and an energy state of crystal grain when the bolometer material is polycrystalline. In this case, for example, the state of the bolometer material is set to the state p1 on the temperature increasing curve in FIG. 5, and next the temperature is decreased so that the state of the bolometer material is set to a state p. Thus, the phase transition can be realized. However, in this case, if the operation temperature is not set to the transition temperature corresponding to the state p1, the sensitive resistivity change along a hysteresis curve can not be realized.
However, the conventional bolometer operates at an ambient temperature. Therefore, even if the physical chemistry structure of the bolometer material is identical with the physical chemistry structure corresponding to the state on the hysteresis curve, the rapid resistivity change along the hysteresis curve can always not be realized.
This problem will be specifically described below.
The temperature resolution (NETD) of the infrared sensor is typically about 0.1xc2x0 C. Therefore, the temperature change of the bolometer due to the infrared rays from a subject is approximately equal to this order. As a result, generally, the temperature change of a heat sensing section is small sufficiently than the temperature width xcex94Tt of the hysteresis. In case of the bolometer material of VO2, it is reported that the temperature width xcex94Tt of the hysteresis is 1xc2x0 C. When the bolometer material is a bulk single crystal, is 2xc2x0 C. when the bolometer material is a polycrystalline film, and is 10xc2x0 C. when the bolometer material is a low crystal film (J. Vac. Sci. Tchnol. A15, 1113 (1997)). Also, it is reported that the temperature width xcex94Tt of the hysteresis is 50xc2x0 C. When the bolometer material is formed of V2O3 with Cr of 1 mol % doped ((Physical Review B 22, 2626 (1980)).
When the infrared rays are incident on the bolometer in the state p (temperature Tobj) of the figure under the above condition, the bolometer resistance value changes as shown by the solid line arrow in the temperature width of the hysteresis. At this time, because the temperature change xcex94Tobj of the bolometer is smaller than the temperature width xcex94Tt of the hysteresis curve, the temperature Tobj of the bolometer does not reach a transition temperature T1 corresponding to the point p1 on the temperature increasing curve. Therefore, the phase transition does not occur. As a result, a high TCR value is not attained and a high infrared ray detection sensitivity is not attained.
Moreover, in a case where the bolometer material having a hysteresis is used for the bolometer, the bolometer cannot be used, if the temperature history is not clear, i.e., it is not clear whether the bolometer material is in the temperature increasing process or in the temperature decreasing process. For example, to realize the state p in FIG. 5, there are two heat-treatment processes.
The first process starts from the state a in the first phase and reaches the state p1 on the temperature increasing curve via the critical state b, and then reaches the state p from the state p1 by decreasing the temperature. The second process starts from the state f in the second phase and reaches the state p2 on the temperature decreasing curve via the critical state d, and then reaches the state p from the state p2 by increasing the temperature.
When the bolometer temperature is increased from Tobj corresponding to the state p attained through the first process, the resistivity xcfx81 (log xcfx81 in FIG. 5) changes gently along the curve p-p1. When the temperature of the bolometer material reaches T1 corresponding to state p1, the resistivity changes rapidly with the temperature increase. However, when the temperature of this bolometer material is decreased from Tobj corresponding to the state p, the resistivity changes gently approximately along the prolongation of the straight line p1-p. At this time, even if the prolongation intersects the temperature decreasing curve, the resistivity change gently without phase transition. This is because only the temperature of the bolometer material changes in the condition in which the physical chemistry structure in the state p1 is fixed. Unless the temperature returns to the state p1 again, the phase transition can not be performed.
When the temperature of the bolometer material is decreased from Tobj corresponding to the state p attained through the second process, the resistivity xcfx81 changes gently along the curve p-p2. When the temperature of the bolometer material reaches the temperature corresponding to state p2, the resistivity is increased rapidly with the temperature decrease. However, when the temperature of the bolometer material is increased from Tobj corresponding to the state p, the resistivity changes gently approximately along the prolongation of the straight line p2-p. At this time, even if the prolongation intersects the temperature increasing curve, the resistivity continues gentle change just as it is without phase transition. This is because only the temperature of the bolometer material changes in the condition in which the physical chemistry structure corresponding to the state p2 on the temperature decreasing curve is fixed. Unless the temperature returns to the temperature corresponding to state p2 again, the phase transition does not occurs.
For these reasons, even if the bolometer materials have the same chemical composition and the same doping density, and are in the same state p, the-bolometers performs contrary operations based on the heat-treatment history. This is the second reason why the bolometer material having a hysteresis can not be used for the bolometer. For the above reasons, there is no report in which the bolometer material having the hysteresis is applied to the bolometer type infrared sensor.
In conjunction with the above description, an infrared sensing apparatus is described in Japanese Laid Open Patent Application (JP-A-Heisei 7-181082). In this reference, an infrared radiating body (29) is provided under a cold side (28) of a thermocouple (25) through an interlayer insulating film to thermally connect the thermocouple to the infrared radiating body (29). A diagnosis circuit (33) applies a voltage to the infrared radiating body (29) for every given time interval or at a given timing to detect thermal electromotive force, whereby to diagnose the thermocouple (25).
Also, a method of controlling electric characteristic of vanadium oxide is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-145481). In this reference, sample wafers (1) having vanadium oxide films (V2O5) with 5 valences are located in a sample holder (2) and subjected to heat treatment using a mixture gas of argon and hydrogen. The heat treatment temperature is in a range of 350 to 450xc2x0 C. In this way, vanadium oxide films (V2O5) with 5 valences is converted into vanadium oxide films (VO2) with 4 valences and then vanadium oxide films (V2O3) with 3 valences. The resistivity of the vanadium oxide is controlled.
Also, a bolometer type infrared sensor is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-257565). In this reference, a vanadium pentoxide is formed by a sputtering method or a sol-gel method and is subjected to heat treatment using a mixture gas of argon and hydrogen. Thus, a vanadium oxide film VOx is obtained, where 1.875 less than X less than 2.0. The vanadium oxide film has no metal-semiconductor transition at about 70xc2x0 C., unlike a typical VO2 film.
Also, a gas detecting apparatus is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-264591). In this reference, the gas detecting apparatus includes a sensor element (102) having a an active film used to detect a gas, and sensing the gas in attribute and density, a heater (103) heating the sensor element, a control unit (101) reading the sensing result of the sensor element to generate a control signal, and a heater adjuster (106) adjusting the temperature of the sensor element. The temperature change of the sensor element is repeated periodically in a predetermined time period or in different time periods. The temperature increase width is different from the temperature decrease width and the temperature change is compared with a hysteresis of temperature control to detect the concentration of the gas.
Also, an infrared sensor is described in Japanese Patent No. 2655101. In this reference, the infrared sensor is composed of a bolometer vanadium oxide film formed on a support film which is thermally isolated from a substrate through a gap between the support film and the substrate, a first protection film of a vanadium pentoxide film formed to cover the bolometer vanadium oxide film, and a second protection film formed a surface portion including t he first protection film .
Therefore, an object of the present invention is to provide a bolometer type infrared sensor in which a bolometer material having a hysteresis in resistivity temperature characteristic is used.
Another object of the present invention is to provide a bolometer type infrared sensor which uses phase transition.
In order to achieve an aspect of the present invention, an infrared ray sensor includes two electrode lines, a heat sensing section and a control unit. The heat sensing section is connected between the two electrode lines, and includes a film formed of a material in which resistivity of the material changes along a hysteresis curve depending on temperature change. The heat sensing section receives an infrared ray to change the resistivity. The control unit is connected to between the two electrode lines. The control unit operates to the heat sensing section such that the heat sensing section undergoes a temperature cycle. The temperature cycle is composed of a temperature increasing process and a temperature decreasing process, and the resistivity of the material changes along a part of the hysteresis curve during the temperature cycle. Also, the control unit detects a temperature due to the infrared ray based on a result of the temperature cycle.
The control unit preferably supplies pulse power to the heat sensing section for every temperature cycle. In this case, the temperature increasing process is performed through heat generation due to the pulse power, and the temperature decreasing process is performed through heat radiation. Also, the control unit supplies pulse power to the heat sensing section such that an inequality (1) is satisfied:
xcex94Tc greater than xcex94Tt+|xcex94Tobj|xe2x80x83xe2x80x83(1)
where xcex94Tc is a temperature change width in the temperature cycle, xcex94Tt is a temperature width of the hysteresis curve, and |xcex94Tobj| is an absolute value of a temperature change due to the infrared ray.
It is preferable that the film is formed of VO2 having oxygen defects, and has a temperature coefficient of resistance equal to or more than 10%/K.
In order to achieve another aspect of the present invention, an infrared ray sensor includes a plurality of heat sensors and a control unit. The plurality of heat sensors are arranged in a matrix, and each of the plurality of heat sensors includes a film formed of a material in which resistivity of the material changes along a hysteresis curve depending on temperature change. The heat sensing section receives an infrared ray to change the resistivity. The control unit operates to each of the plurality of heat sensors such that each heat sensor undergoes a temperature cycle. The temperature cycle is composed of a temperature increasing process and a temperature decreasing process, and the resistivity of the material changes along a part of the hysteresis curve during the temperature cycle. The control unit detects a temperature of the each heat sensor due to the infrared ray based on a result of the temperature cycle.
The control unit supplies pulse power to the each heat sensor for every temperature cycle. In this case, the temperature increasing process is performed through heat generation due to the pulse power, and the temperature decreasing process is performed through heat radiation. Also, the control unit supplies pulse power to the each heat sensor such that an inequality (1) is satisfied:
xcex94Tc greater than xcex94Tt+|xcex94Tobj|xe2x80x83xe2x80x83(1)
where xcex94Tc is a temperature change width in the temperature cycle, xcex94Tt is a temperature width of the hysteresis curve, and |xcex94Tobj| is an absolute value of a temperature change due to the infrared ray.
It is preferable that the film is formed of VO2 having oxygen defects, and has a temperature coefficient of resistance equal to or more than 10%/K.
In order to achieve still another aspect of the present invention, a method of detecting a temperature due to an infrared ray, includes:
making a heat sensing section undergo temperature cycles, wherein the temperature cycle is composed of a temperature increasing process and a temperature decreasing process; and
detecting a temperature due to the infrared ray based on a result of each of the temperature cycles.
To make a heat sensing section undergo temperature cycles, pulse power is supplied to the heat sensing section for every temperature cycle. In this case, the temperature increasing process is performed through heat generation due to the pulse power, and the temperature decreasing process is performed through heat radiation.
The supply of the pulse power to the heat sensing section is performed such that an inequality (1) is satisfied:
xcex94Tc greater than xcex94Tt+|xcex94Tobj|xe2x80x83xe2x80x83(1)
where xcex94Tc is a temperature change width in the temperature cycle, xcex94Tt is a temperature width of the hysteresis curve, and |xcex94Tobj| is an absolute value of a temperature change due to the infrared ray.
It is preferable that the heat sensing section is formed of VO2 having oxygen defects, and has a temperature coefficient of resistance equal to or more than 10%/K.
In order to achieve yet still another aspect of the present invention, an infrared ray sensor includes two electrode lines, and a heat sensing section connected between. said two electrode lines, and thermally separated from a peripheral object. The heat sensing section includes a supporting film, and a film formed of a material having a hysteresis in a resistivity temperature characteristic.